Most modern food dehydrators are low-power convection ovens that uses heated air flow to reduce the water content of foods. The water content of food is usually very high, typically 80–95% for various fruits and vegetables and 50–75% for various meats. Removing moisture from food restrains various bacteria from growing and spoiling food. Further, removing moisture from food dramatically reduces the weight and often volume of the food, making it easier for storage. Thus, food dehydrators are used to preserve and extend the shelf life of various foods.
Food dehydrators require heat sources such as solar energy, electric power or biofuel, and vary in form from large-scale dehydration projects to do-it-yourself projects or commercially sold appliances for domestic use. A commercial food dehydrator's basic parts usually consist of a heating element, an electric fan, air vents which allow for air circulation, and food trays to lay food upon. A dehydrator's heating element, fans and vents simultaneously work to direct hot air over the food, accelerate surface evaporation and warm the food causing moisture to be also released from its interior. This process continues until the food is dried to a substantially lower water content, usually less than 20%.
Most foods are dehydrated at 130 °F (54 °C), although meats being made into jerky should be dehydrated at a higher temperature of 155 °F (68 °C) — or preheated to that temperature — to guard against pathogens that may already be in the meat. The key to successful food dehydration is the application of a constant temperature and adequate air flow. Too high a temperature can cause hardened foods: food that is hard and dry on the outside but moist, and therefore vulnerable to spoiling, on the inside.
Solar food dehydrators
Solar food drying involves the use of a solar dryer designed and built specifically for this purpose. Solar drying is distinctly different from open-air "sun drying", a simple technique that has been used for thousands of years. A good solar food dryer may dry food much faster than some air driers. Some solar driers can achieve higher food drying temperatures than some air driers. However, direct sun drying can chemically alter some foods making them less appetizing.
Food drying is an excellent solar energy application since food drying primarily requires heat, and solar radiation is easily converted to heat. A clear or translucent glazing allows sunlight to enter an enclosed chamber where it is converted to heat when it strikes a dark interior surface. Airflow is typically achieved with natural convection (warm air rises). Adjustable venting allows regulation of airflow and temperature.
Solar food drying is effective and practical in most of the populated places of the world. A general rule is that, if you can grow a successful vegetable garden, then there is enough solar energy to dry the food you produce (some overcast, northern maritime climates are the exception).
Some solar food dryer designs employ a separate solar collector to generate the heated air, which is then directed into a food chamber or cabinet. This type of solar food dehydrator is called an indirect solar dryer. Other designs combine the collector and food cabinet and allow direct heating of food (these are called direct solar dryers). It is said that the indirect dryers allow much better drying, but the downside is their larger size. Backup electric heating can be incorporated into some solar food dehydrators to provide an alternative heat source if the weather changes.
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